The concerted attack by rebels on al-Qaeda across northern Syria could be regarded as a step in the right direction by the West

A concerted attack by rebels across northern Syria on the most ferocious of the al-Qaeda groups involved in the war has been long awaited.

The Islamic State of Iraq and al-Sham (ISIS), which has in its ranks hundreds of British and other foreign jihadis, managed to offend just about everyone: civilians on whom it inflicted its aggressively puritan morals, up to banning cigarettes, secular rebel groups it picked fights with, and even other Islamist militant leaders, one of whom it accidentally beheaded in a dramatic case of mistaken identity.

Taking ISIS on is nevertheless a major risk for these rebels, who comprise secular fighters still vaguely loyal to the original pro-democracy, western-aligned goals of the revolution and Islamist groups whose militant anti-western ideology is not so far removed from that of al-Qaeda itself.

If the attack does not work, they will face ferocious retribution. Even if it does succeed in destroying ISIS, the fighting between the two sides will be a boon to the Assad regime, already poised to capitalise.

However, the rewards could be even bigger. The rise of ISIS has becalmed the armed opposition inside Syria. Western governments will no longer touch it, and have suspended what little quasi-military aid they were sending to Assad's opponents.

Saudi Arabia and other Gulf states, for all their diplomatic bluster and claims to be backing the rebels to the hilt, have also been less than regular in sending the heavy arms they need to make an impression on the forces of the regime and its Hizbollah allies.

By kidnapping aid workers and journalists, presumably to use as hostages in case of some future stand-off with the West, ISIS also seriously damaged the wider opposition more directly, depriving civilians of aid supplies and the cause more generally of the oxygen of publicity.

The fight against ISIS is being led by the most important single rebel outfit in northern Syria, Ahrar al-Sham, and its allies in the Islamic Front. These are no friends of the West, though Britain and America have made tentative approaches, and an opposition led by them would be no more likely to receive American weapons than before.

They too want an Islamist state: they just do not see their fight as crossing borders, as ISIS does.

But a more unified Sunni opposition would be regarded by the West as a step in the right direction, and by its backers in the Gulf as an unalloyed advantage. It would also mean that when the West faces off against Russia in forthcoming peace talks about which, at the moment, the people actually fighting seem to have little to say, the opposition's backers will at least have a chip with which to bargain.